Calcium is the most abundant mineral in the human body and is essential for building and maintaining bones and teeth. Only 20-30% of the calcium in milk can be absorbed by the human body. 90% of the insoluble calcium in milk is located in the casein micelles, and 40% of this calcium is hard to exchange.
It is important to understand how this non-exchangeable calcium behaves and whether there are ways of making it exchangeable.
In this study, the hard-to-exchange (HTE) calcium in the casein micelles of bovine skimmed milk was studied. In order to investigate this, a protocol for the preparation of artificial casein micelles was established. The hydrodynamic radius and the hydration of the casein micelles were measured to see if the artificial casein micelles could be compared to the casein micelles from bovine skimmed milk. Subsequently, 45Ca was used to track the exchange of calcium in the three different fractions of milk and artificial casein micelles: casein micelles, serum proteins, and soluble phase. (Ultra)centrifugation was used to separate the fractions.
Characterisation of bovine skimmed milk and artificial casein micelles was carried out first. The hydrodynamic radius of the casein micelles in both types of milk was determined and it was concluded that the hydrodynamic radius of both types of milk was comparable to each other. Furthermore, the hydration of casein micelles was investigated. A difference was measured between the hydration of both casein micelles. This can be explained by a small deviation in casein and fat content between the two types of milk.
In order to draw conclusions about the suitability of using artificial casein micelles to model casein micelles naturally present in milk, the protocol needs to be adapted by adjusting the casein content and removing the fat from both types of casein micelles. It is also necessary to investigate the mineral content of both types of milk.
It was found that almost half (48.8 ± 0.6%) of the 45Ca added to bovine skimmed milk was located in the supernatant, while the remaining 54.2 ± 0.6% was distributed within the casein micelles. When this distribution is compared to that of artificial casein micelles, it can be seen that the %45Ca in the artificial casein micelles is much lower than in the bovine skimmed milk casein micelles. This is because of the fact that artificial casein micelles contain a lower content of casein aggregates cross-linked with colloidal calcium phosphate (CCP) in comparison with bovine skimmed casein micelles.
When 45Ca was added after the preparation of artificial casein micelles, the HTE calcium was found to be 54.5 ± 2.6%. However, when the 45Ca was added during the preparation of artificial casein micelles and measured after 0 hours, the HTE calcium was 63.1 ± 2.0% and after 24 hours it was determined to be 59.6 ± 3.4%. These values were not in agreement with the values from Zhang et al. [1].
Overall, this project contributed to a better understanding of the hard-to-exchange calcium in milk using artificial casein micelles and 45Ca. It shows that further research needs to be carried out to determine whether artificial milk can be used as a model for bovine skimmed milk and to determine the HTE calcium in artificial casein micelles.